Effect of times, height and frequency of defoliation on growth and development of Townsville stylo in pure ungrazed swards at Katherine, N.T

Abstract

A one-year experiment was done in the field at Katherine, Northern Territory, to determine the influence of defoliation at different times (January, February, and March) and heights (5, 13 and 20 cm) on the growth and development of Townsville stylo (Stylosanthes humilis) in ungrazed swards grown with complete fertilizer and irrigation. Townsville stylo withstood repeated defoliation at 5 cm provided that cutting started in January, but, when the first defoliation was delayed, single cuts at 5 cm in either February or March killed 82 and 53 per cent of the swards. Defoliation to 5 cm early in the season reduced total cumulative yield by less than 450 kg ha-1, and did not affect pod yield or nitrogen and phosphorus concentrations at the end of the growing season. Immediately after defoliation nitrogen and phosphorus concentrations were reduced, but there was rapid recovery to higher levels with 5-cm and the same levels with 13-cm defoliation. Successive defoliation at 4 3 cm in February and March increased yield of pods. Defoliation at both 5 and 13 cm increased relative growth rate compared with the control and less frequently defoliated treatments. Defoliation to 5 cm each four weeks altered the stand morphology to a dense mat of foliage at cutting height. The ability of Townsville stylo to withstand continued defoliation suggests that sustained grazing during the growing season may be successful in controlling grass weeds vulnerable to grazing without greatly reducing pod yield or total dry matter production of Townsville stylo.